This invention relates to radio communications systems. In particular this invention relates to trunked radio systems and specifically to a method and apparatus for extending the geographic coverage area provided by a trunked radio system.
Trunked radio systems provide limited range two-way radio communications to subscriber radios that are within the geographic coverage area of a repeater station for the system. Subscriber radios of a trunked radio system time-share a limited number of radio communication frequencies with other radios, including other unrelated radios under the direction of a controller for the system. The central controller temporarily assigns and re-assigns available frequencies to certain radios based on a variety of factors, which are irrelevant to this invention.
When a subscriber radio requests a communication channel, it sends a request for a communication channel to the resource controller for the system via a control channel. (The control channel is a radio channel that carries trunked radio system commands and data. The resource controller continuously send outbound signalling words (OSW's) to the subscriber radios and the subscriber units send inbound signalling words (ISW's) to the resource controller on the control channel when they request service. The control channel, as well as the communication channels for the trunked radio system, are referred to hereafter as a control resource and communication resources respectively since the control channel and communication channels might include distinct frequency bands using distinct modulation techniques as well as a particular time slot in a TDM frame or other uniquely identifiable channel for example.) This controller, which is more properly called a communication resource controller and typically includes a computer, receives requests for communication resources from subscriber radios via ISW's on the control resource and, using OSW's, directs subscriber radios to shift to a particular frequency and monitor frequency for communications traffic.
Large trunked radio systems might include multiple repeaters distributed around a geographic region to expand the area over which communication can be maintained. The repeaters broadcast and receive the OSW's and ISW's as well as the communications information. To expand the geographic coverage area, a repeater is usually added in or near the region where extended communication is desired. If the new repeater is beyond the range of the existing system, it must be linked to the resource controller by either a dedicated microwave link, dedicated phone line or other communication path. Microwave links and phone lines can be expensive to install and maintain however, and a method and apparatus for expanding the geographic coverage area of a trunked radio system that reduces the cost of prior art range expansion techniques would be an improvement over the prior art.